Micellization of Model Macromolecular Surfactants as Studied by Static Light Scattering

Abstract

Micellization of ionically end-capped polystyrene-block-polyisoprene copolymers (PS-b-PI) in N,N-dimethylacetamide (DMAc) is investigated by static light scattering. DMAc is a polar selective solvent for PS which ensures for sufficient dissociation of ion pairs in solution. The use of polyisoprene as the nonsoluble block provides micelles in thermodynamic equilibrium since PI is a liquid at ambient temperature. Two different types of ionic end functionality are considered: (i) a monofunctional diblock with a lithium sulfonate group at the PI chain end and (ii) an α,ω-macrozwitterionic species with an additional ammonium group at the PS chain end. They are both compared to a neutral diblock of same mass and block sizes (12 kg/mol per block). It is shown that micelles formed by block copolymers containing a sulfonate group at the PI end (core of the micelle) exhibit an aggregation number (Nagg) which is only about 30% of Nagg of the neutral block copolymer. The critical micelle concentration (cmc) in this case is increased by 1 order of magnitude. The macrozwitterionic samples behave qualitatively as the monofunctionalized polymer, however, presenting a larger aggregation number and a lower cmc with respect to the monofunctionalized sample. This effect is attributed to a self-screening mechanism in micelles of the α,ω-macrozwitterions, where counterions are bound through chains of the soluble block.